The present invention relates to thermoplastic, ethylenic polymer foams, more particularly, to closed cell, low density ethylenic polymer foams prepared using inorganic halogen-free blowing agents and to a process for the preparation thereof.
It is well known to prepare ethylenic polymer foams by heat plastifying a normally solid ethylenic polymer, admixing, under heat and pressure, such heat plastified polymer with a volatile organic blowing agent to form a flowable gel, and, thereafter, extruding the gel into a zone of lower pressure and temperature to expand and cool the gel to form the desired solid ethylenic polymer foam product.
A phenomenon frequently encountered in the preparation of ethylenic polymer foams is that of changes in foam dimensions (volume) which occur during the aging or curing period following manufacture. During the aging or curing period, the blowing agent employed gradually diffuses out of the cells in the foam product and air gradually diffuses into the cells in place thereof.
The conventional blowing agents used in the production of these low density ethylenic polymer foams are generally a high percentage, if not entirely, organic blowing agents including fluorocarbons, such as saturated chlorofluorocarbons (CFC's) and hydrochlorofluorocarbons (HCFC's) and other halogenated hydrocarbons. Such chlorofluorocarbons are suspected of destroying the earth's protective ozone layer. Non-halogenated hydrocarbons may also be employed as blowing agents. The non-halogenated hydrocarbons do not damage the ozone layer, but are highly flammable.
Due to increased environmental concerns about ozone depletion, greenhouse effects and air quality in general, large efforts are being made to replace CFC's, HCFC's and other halogenated hydrocarbons currently used as blowing agents in the foam industry with environmentally more acceptable blowing agents. Most of the replacements are the non-halogenated hydrocarbons (sometimes in mixtures with carbon dioxide), but these still present hazards during foam manufacture as well as during application and/or fabrication due to their high flammabilities.
One potential alternative that has been considered and evaluated is the use of inorganic blowing agents, such as carbon dioxide or argon, as the sole blowing agents. However, compared with the conventional blowing agents, both carbon dioxide and argon present numerous difficulties due to their relatively low solubilities in the ethylenic polymer, low heats of vaporization and, in the case of carbon dioxide, high permeability through ethylenic polymers. Hence, previous ethylenic polymer foams produced with carbon dioxide or argon as the sole blowing agent have been restricted to sheet products and/or foams having densities greater than 150 kg/m.sup.3, and mostly in sheet form.
Lower foam densities were only achieved by crosslinking to obtain adequate melt strength to prevent cell collapse during the foam expansion. However, crosslinked foams cannot be recycled, hence, they are environmentally less accepted.
Therefore, it is still highly desirable to develop techniques of using blowing agents which have the potential for less environmental damage and which are non-flammable and which result in ethylenic polymer foams having properties as close as possible to those prepared with conventional blowing agents. More particularly, it is highly desirable to provide ethylenic polymer foams having densities below 150 kg/m.sup.3 which have been blown with carbon dioxide as the single blowing agent, without the need of crosslinking.